1. Field of the Invention
This invention relates to a method of manufacturing a multilayer ceramic body, such as a multilayer ceramic substrate, which mounts thereon semiconductor ICs and chips and interconnects them.
2. Description of the Prior Art
The multilayer ceramic substrate manufacturing methods are classified into three: a thick film method, a green tape printing method and a green tape laminating method.
The thick film method is represented by the hybrid IC, which employs thick film paste of conductors and dielectric and repeatedly applies to an already sintered ceramic substrate the screen process printing and firing to thereby form patterns. This method is relatively easy because the thick film paste is readily obtainable and the method itself is simple. Thus, this method is now widely in practical use. The thick film method, however, uses glass as the dielectric layer and is not so much multilayered, being limited to three to four layers. The screen process printing and firing are repeated for multilayering each layer to result in a long lead time and a high manufacturing cost. Also, the use of the sintered substrate will create a defect in that the through hole processing is difficult.
Next, the green tape printing method uses a ceramic sheet which is fabricated by the following process. A ceramic powder (e.g. alumina or beryllia as the main component) added with an organic binder, a plasticizer and a solvent, formed in slurry by use of a ball mill, and formed in a sheet-like film (called the green tape) by means of the doctor blade. The conductor paste uses high melting point metal, such as tungsten (W), molybdenum (Mo) or molybdenum-manganese (Mo-Mn), and the dielectric layer paste uses paste of an inorganic component having the same composition as the green tape material. In this method, the conductor paste and dielectric paste are printed alternately on the green tape to be multilayered, and after printing and drying, sintering is carried out at once. The sintering is usually carried out under a reducing atmosphere in which the high melting point metal is not oxidized. For example, 96% alumina is sintered at a temperature of 1600.degree. C. The reducing atmosphere usually contains nitrogen and some water vapor gases including hydrogen gas of concentration of about 10% (as disclosed in, for example, "A FABRICATION TECHNIQUE FOR MULTILAYER CERAMIC MODULES", Solid State Technology 15, No. 5, P 35.about.40 (May, 1972)).
Such green tape using method is very advantageous and expected to be widely used in the future. Its advantages are as follows: Firstly, the sintering is carried out at once after the tape is printed and multilayered to thereby reduce manufacturing time. Secondly, the insulating layers are the same in composition as the substrate material and are sintered simultaneously so that a dense sintered body superior in thermal conductivity and airtightness is obtainable. Thirdly, the use of green tape increases processability, such as the through hole processing and is superior in fine printing efficiency. Fourthly, the use of metal, such as W, Mo or Mo-Mn, lowers a material cost in comparison with Au or Ag/Pd series conductor material. Fifthly, the sintered body is shrunk when sintered, thereby being actually higher in fine line printing. Sixthly, adhesion strength of conductor layer is larger than that in the thick film method.
The green tape method, however, is defective in that a large design change is difficult; it is dangerous to be processed at a high temperature and to require a hydrogen atmosphere to result in a high fabrication cost; and the conductor is higher in electrical resistance than Au, Ag or Cu and not to be soldered, thereby requiring the surface treatment for plating Ni or Au on the surface.
The green tape laminating method is similar to the green tape printing method, but different in the multilayering process of laminating a number of green tape printed conductors and formed through holes.
This method is effective for a large number of laminations, but requires molds or jigs as many as the laminations for the through hole processing to the green tape and is low in a degree of freedom for design change, thereby having been not as popular as the green tape printing method.
Next, paying attention to the metallized conductive material used for the ceramic substrate, the thick film method uses Au, Ag/Pd or Cu, and the green tape method, W, Mo or Mo-Mn. The Au and Ag/Pd are fired in air, but this method is expensive due to noble metals. Also, the green tape method, which sinters the ceramic substrate at a high temperature of 1500.degree. C. or more, has a problem in that only high melting point metal, such as W, Mo, or Mo-Mn, is usable. Hence, at present a Cu metallized substrate has been noted which is low in conductor resistance, generates no migration phenomenon, and is good in soldering. So, the Cu metallized wiring substrate is already put into practical use.
But, there is a drawback in the Cu metallized substrate because of using the base-metal. The reason for this is that the base metal, when fired in air, is oxidized not to obtain the conductivity, and that in order to obtain the adhesive property sheet resistance, and soldering property of the wiring and to eliminate decomposition of organic binder in the paste, very delicate atmosphere control including some oxygen in the nitrogen atmosphere is required.
Furthermore, in a case where a glazed resistor and a dielectric are formed after the Cu conductor is formed, the firing atmosphere as the same as the above-mentioned is required. However, such resistor and dielectric usable in this condition are rare and the degree of freedom for selection is very little. Nevertheless, the merit of the base metal conductor material represented by Cu is attractive.
Now, in consideration of the future of the multilayer substrate, it will be ideal to use the base metal material as the conductor material and to utilize the green tape method for the multilayer method. In other words, a base metal conductor, such as copper, is printed on the green tape and the insulating layer is printed or laminated in multilayers so as to obtain the multilayer substrate.
However, there are some problems to be solved in order to put the base metal multilayer substrate into practical use. A first problem is created in that since the melting point of Cu, the typical base metal, is low at a temperature of 1083.degree. C., it is necessary for sintering simultaneously with the substrate material to keep the sintering temperature thereof lower than the aforesaid melting point. It is indispensable to satisfy mechanical strength of sintered body, break-down voltage, moisture resistance and thermal conductivity required to the substrate material, and further the performance, such as metallizing property of Cu, is required when multilayered. A second problem is that it may be difficult to use the binder under such sintering condition of temperature or atmosphere. In other words, the organic binder used for the sheet or the paste has a property of non-decomposition in a nonoxidizing atmosphere. Unless the binder is completely decomposed and removed, the ceramic body itself remains porous, whereby not only the sintering does not proceed but also the substrate becomes blackish due to the residual carbon.
For the aforesaid simultaneously sintering substrate material, glass-ceramic material has generally been developed, which is disclosed in, for example, U.S. Pat. Nos. 3,977,887 and 4,301,324. The decomposition and removal of the binder is disclosed in Japanese Laid-open Patent Application No. 55-128899.